Environmental control systems for dwellings and commercial establishments invariably include heating and air conditioning systems for modulating the temperature of the enclosed air. Furthermore, such systems also include filtration means for removing particulate matter from the heated or cooled air. Such filtration means, particularly for dwellings, include mechanical filters formed of entwined or interwoven fibrous materials.
These fibrous materials serve the function of mechanically blocking flow of particulate matter as the air flowing therethrough flows along a circuitous path. The particulate matter becomes attached to individual fibers and, over a period of time, progressively restricts the air flow paths. The pressure differential between the inflow and outflow sides of the air filter increases as a function of the quantity of particulate matter filtered by the filter and retained on the air filter fibers. Simultaneously, a reduction of the air flow rate through the filter occurs.
The effects of a contaminated or partially clogged air filter are many-fold. The air flow rate is diminished which reduces the effective heating or cooling of the dwelling or commercial establishment. The partially clogged filter tends to increase the back pressure applied to the blower or fan which generates the air flow; such back pressure increases the work which must be performed and the energy consumed by the blower or fan unit. The increased work load increases the wear rate and operating costs increase.
Various devices have been developed over the years to provide a determination or reminder to a user of the need for replacement of an air filter in an heating and air conditioning unit. In each of U.S. Pat. Nos. 2,753,831 and 4,321,070 there is describd a tube which extends through an air filter and incorporates a whistle. In operation, air flows continuously through the tube and as the air flow threthrough increases as a result of increasing clogging of the air filter, the whistle generates a sound when the air flow rate is of a sufficient magnitude. As air flows continuously through the tube, some contamination and clogging of the tube will inherently occur and may have a negative effect upon the operation of the whistle. U.S. Pat. No. 2,804,839 is directed to a device for providing a visual and audible signal indicative of the clogging of an air filter. The device is a highly complex mechanical device; it includes a magnet for retaining in place a pivotable member which member provides a visually perceivable indication and actuates further structure capable of energizing an audible alarm. Moreover, the device is self-resettable as a result of complex interrelated mechanical movements which become operable upon replacement of the air filter. U.S. Pat. No. 3,916,817 is directed to a pivotally mounted disc movable against the action of a spring in response to a pressure differential of a predetermined value to provide a visually perceivable indication reflective of repositioning of the disc. U.S. Pat. No. 3,027,865 is directed to a device having a diaphragm responsive to creation of a partial vacuum for providing a visual indication and for closing a pair of contacts to energize an electric signal. U.S. Pat. No. 2,721,533 is directed to an air blower wall mounted alarm having a continuous flow of air therethrough during normal operation; upon an increase in air flow resulting from a clogged air filter, a vibrating reed becomes energized to generate an audible signal, The operating threshold may be modified by adjusting the air inlet size. U.S. Pat. No. 2,782,747 is directed to a device penetrably attached to the wall of a blower system for providing a reed generated audible signal upon an increased air flow therethrough due to a clogged air filter associated with the blower system. U.S. Pat. No. 3,736,900 is directed to a reed operated device for producing an audible signal as a result of clogging of an air cleaner filter associated with the carburetor of an internal combustion engine.